1. Field of the Invention
The present invention relates to a method of cutting a workpiece such as a thick plate of mild steel, for example, with a laser beam, and more particularly to a method of cutting a workpiece with a laser beam which is focused with improved focussing characteristics that would otherwise be degraded by optical distortion.
2. Description of the Related Art
Conventional CO.sub.2 laser cutting machines mainly have had a laser beam output of 1 KW or less, and have been able to cut workpieces of mild steel, for example, which are up to 9 mm thick. In the above power range, the primary technical concern has been how small a focus spot can be made by focussing laser beam. The focussing a characteristics of the laser beam are affected by barious factors such as the mode order that determines the divergent angle of the laser beam, the laser beam diameter on the focussing lens which determines a diffraction limitation, and the aberrations of the focussing optical system. Particularly, the mode order has been considered to be most important for the CO.sub.2 laser cutting machines, and the achievement of the TEM00 mode which is the lowest-order mode is given the utmost importance. The TEM00 mode has the smallest divergent angle, and has the best focussing characteristics which make fine machining possible.
The above approach has also been employed for cutting machines with high-power CO.sub.2 lasers. While attempts to produce higher-power CO.sub.2 lasers. While attempts and attempts to achieve the TEM00 mode are generally contradictory, various efforts have been made to lower the mode order.
However, we have found, through experimental and theoretical analysis, that in output power ranges higher than 2 KW, the focussing optical system suffers optical distortion in the mode primarily of TEM00, and behaves in a manner completely different from that when the output power is lower, making it impossible to focus the laser beam into a small beam spot.
The above drawback manifests itself especially with a ZnSe lens. More specifically, the lens undergoes a temperature rise due to laser beam absorption. Since the power density in the central region of the lens greatly increases in the TEM00 mode, the temperature distribution becomes similar to the power density distribution. As a result, the lens region which suffers the high temperature is thermally expanded and has an increased refractive index, resulting in a localized change in the focussing characteristics.
The degradation of the focussing characteristics in the TEM00 mode goes far beyond its combined inproving effect of the three factors that affect the focussing characteristics. Therefore, the prior approach to improve the focussing characteristics has proven disadvantageous in the output power ranges beyond 2 KW.